1. Field of the Invention
The present invention relates to an image forming apparatus and a method for image forming and a program thereof and, particularly, to gradation correction when forming an image.
2. Description of the Related Art
The mainstream of the recent copy machine is a digital copy machine that performs digital processing on image data generated by reading a document with a scanner unit, transfers the image data to a printer unit, and prints the transferred image data. In the copy machine with such a configuration, if the gradation property of the image data generated by the scanner unit and the gradation property when output to a sheet in the printer unit are not adjusted, the gradation property in the duplicated document will be lost as a result. In view of this, various automatic gradation correction functions are proposed which adjust the gradation property in a duplicated document to be output when the document is duplicated.
The automatic gradation correction function is explained with reference to FIGS. 4 and 5. First, an image A 510 is created as a gradation measurement chart by a controller 501 of a copy machine and is stored in a memory within the controller 501 (steps 401 to 403). Then, the image A 510 stored in the memory is transferred to a printer unit 503 connected to the controller 501 and printed in the printer unit 503 and a printed document A 512 is output (step 404).
Next, the printed document A 512 is read by a scanner unit 504 connected to the controller 501 and an image A′ 511 that is obtained is accumulated in the memory (steps 405 to 407). At this time, it may also be possible to accumulate predetermined target data in the memory in place of the image A′ 511. The image A 510 and the image A′ 511 (or target data) saved in the memory within the controller 501 are compared, and thus gradation difference data is calculated (step 408). Each process of steps 403 to 408 is repeated several times (step 409), and thereby a conversion table for gradation correction used at the time of output to the printer unit 503 is obtained (step 410).
The image data read by the scanner unit is converted by use of the gradation correction conversion table created in this manner, which makes it possible to reproduce the gradation of the original document when printing is performed in the printer unit 503. As the automatic gradation correction function, in addition to the method described above, there is a correction method that takes into consideration the difference between sheets used at the time of printing and such an automatic gradation correction function is described in Japanese Patent Laid-Open No. 2001-119594.
The above-described automatic gradation correction is based on the premise that printed matter is always output with the same printing characteristic when image data is printed in the printer unit. However, in general, the printing characteristic of the printer unit varies depending on the number of printed sheets and the printing interval and also varies depending on the use environment. As a result, there is a case where a variation in density occurs between the first several sheets and the several subsequent sheets of the printed matter. The reason is that the fixing temperature, when printing is performed in the printer unit, varies depending on the weather condition of the installation position, the printing time, etc., and therefore, it is no longer possible to perform gradation correction accurately. Consequently, in order to perform gradation correction more accurately, it is desirable to perform printing in a state in which variation in density due to the variation in the fixing temperature is suppressed.